The present invention relates to optical pickup devices for recording and reproducing optical information recording media, and to an object lens unit used in such optical pickup devices.
In recent years, with the practical application of short wavelength red semiconductor lasers, the Digital Versatile Disk (DVD) which is a high density optical disk having a large memory capacity and being of about the same size as a CD (Compact Disk), which is a conventional optical disk (also called an optical information recording medium) has been developed and commercialized.
Following this, optical pickup devices have been developed that are compatible with several types of optical information recording media (specifically, CDs and DVDs). As a pickup of this type, known is one (for example, see Patent Document 1) that features a reduced size and is simpler by using lasers with different wavelengths depending on the type of optical information recording medium, forming diffracted light using diffraction optical elements, and focusing the diffracted light of different orders as information recording light or information reproducing light depending on the wavelength by using a condenser lens. In addition, a phase control element (diffraction optical element) and an object lens (condenser lens) are fixed by a holder within the pickup device, and in particular, a type has been disclosed (see Patent Document 2) in which both the phase control element and the object lens have respective positioning marks to adjust the central axes.
On the other hand, in the near future, still next-generation optical disks of higher density are expected to be commercialized. In the condensing optical systems of optical pickup devices using such next-generation optical disks as a medium, in order to achieve high density of recording signals, or in order to reproduce high-density recorded signals, it is required to reduce the diameter of the focused spot on the information recording surface using an object lens. This requires shorter laser wavelengths, which is a light source, and a higher numerical aperture number of the object lens. A blue-violet semiconductor laser of a wavelength of approximately 400 nm is the laser that is expected to soon be practically applied as a short wavelength laser light source.
As an example of the research and development of high-density optical disk system that can carry out recording and reproduction of information using such a short wavelength laser light source, well-known ones are an optical disk that can carry out information recording and reproduction with a specification of an NA (Numerical Aperture) of 0.85 and a light source wavelength of 405 nm (such as the Blu-ray Disk), and an optical disk that can carry out recording and reproduction of information at a specification of an NA of 0.65 and a light source wavelength of 405 nm (for example, the HDDVD). (Hereinafter, all optical disks with a specification of a light source wavelength of 405 nm are collectively referred to as “High-Density DVD”.)
Following the development of such high-density DVDs, a lens unit has been proposed for an optical pickup unit that integrates a diffracting optical element and a focusing lens and that carries out recording and reproduction of information from three types of optical information recording media conforming to different standards such as high-density DVD, normal DVD, and CD (see Patent Document 3).                Patent Document 1: Tokkai No. 2001-93179        Patent Document 2: Tokkai No. 2001-6203        Patent Document 3: Tokkai No. 2004-62971        
However, in the technologies disclosed in Patent Documents 1 and 2, there still remain the problems of alignment of various optical elements that are the constituent elements of the optical system in the manufacturing of condensing optical system for optical pickups. For example, in Patent Document 1, although it has been described that the condensing lens and the diffraction optical element are placed coaxially with the optical axis by a holder, a specific clear description has not been given regarding the positioning in the direction of the optical axis nor about any concrete method for the coaxial placement. In addition, in Patent Document 2, although there is description of alignment using positioning marks on the phase control element (diffraction optical element) and on the object lens (the condensing lens), there is no description of the positioning related to the direction of the optical axis. Further, in both Patent Documents 1 and 2, each of the optical elements is mounted directly as a separate component onto the holder which is a part of the optical pick-up device. In other words, the accuracy of relative positioning between the different optical elements and the accuracy of alignment of each optical element based on the holder is extremely important in order to increase the image forming performance in the condensing optical system. However, in the above technologies, the work becomes complicated because the work of relative positioning between the different optical elements and the work of their alignment relative to the holder are incorporated in the assembly process of the optical pickup device, whereby it is difficult to obtain high accuracy of condensing or image formation by a condensing optical system comprising two optical elements.